1. Field of the Invention
The present invention relates to a display and drawing control system which delivers the content of display on a display unit, such as a CRT unit, also to a printer.
2. Description of the Prior Art
FIG. 1 is a block diagram of the conventional display and drawing system. FIG. 1 includes a central processing unit 1 (termed CPU hereinafter), a memory 2, an interface circuit 3 (will be termed I/F circuit hereinafter) for a printer 4, a CRT controller 5, a CPU bus 6 led out of the CPU 1, a set of signal lines 7 connecting the printer 4 to the I/F circuit, a frame memory 8, a CRT display unit 10, a CRT bus 11 connecting the frame memory 8 to the CRT controller 5, a set of signal lines 12 connecting the CRT display unit 10 to the frame memory 8, and an input/output unit 14 connected to the CPU 1.
Next, the operation will be described, starting with the CRT display unit 10. Among the various frames displayed, it is assumed that the CPU 1 produces a frame m. The CPU 1 issues various commands for producing the frame m to the CRT controller 5. Then, the CRT controller 5 responds to the commands to store figures or characters in the frame memory 8. Namely, frame data which constitutes the frame m is stored in the frame memory 8. The stored frame data is read out of the frame memory 8 and displayed on the CRT display unit 10.
In applications such as factory automation (FA), the need frequently occurs to display the operational states of a plant on the CRT display unit 10 and, at the same time, to make a hardcopy of the display. For example, actual results such as production output are displayed on the CRT display unit 10 and data in the same form as on the screen is recorded on the printer 4.
In order to make a hardcopy of the frame m, the CPU 1 controls to store the data in the frame memory 8 in the same procedure as the above-mentioned case, and then the CPU 1 operates on the CRT controller 5 to transfer the contents of the frame memory 8 to the memory 2. The contents of the memory 2 are sent to the printer 4, one block at a time, so that a hardcopy of the frame m is produced.
However, in the foregoing display and drawing system, if it becomes necessary to make a hardcopy of frame m while another frame n is being monitored, the frame n must be erased to display the frame m, and after the frame m has been copied the frame n must be displayed again. Accordingly, monitoring the frame n is suspended at least for a period after the commencement of display of the frame m until the termination of copying of the contents of frame m to the memory 2. This time period is considerably long (10 seconds to 2 minutes) and can be a problem in the FA field in which a quick response is required.
Since a mass of frame data occupying the CRT bus 11 and CPU bus 6 in transferring data to the printer 4, results in an increased burden of the CRT controller 5 and CPU 1, there arises another problem of slower throughput of the whole system.